340,147 research outputs found
On Multi-stack Visibly Pushdown Languages
No full textWe contribute to the theory of formal languages of visibly multistack pushdown automata (MVPA). First, we show closure under the main operations and decidability of the main decision problems for the class of MVPA restricted to computations where a symbol can be popped out of a stack S only if it was pushed within the last k contexts of S, for a given k (in a context only one stack can be pushed or popped). In particular, this class turns out to be determinizable. Second, we show the closure under complement of the class of languages accepted by ordered MVPA (where the limitation is that a stack can be popped only if all the lower indexed stacks are empty). This gains decidability of universality, inclusion and equivalence for this class. As a further contribution, we compare the classes of languages accepted by different models of MVPA
Design improvement of circular molten carbonate fuel cell stack through CFD Analysis
No full textMolten carbonate fuel cell (MCFC) is a promising technology for distributed power generation. The core of an MCFC power generation unit is the stack, where various fuel cells are connected together in series and parallel in order to obtain the desired voltage and power. Stack geometry and configuration are major engineering topics, as inhomogeneous temperature or mass fractions cause inefficient performances of the fuel cells, as efficiency and power smaller than the expected and shorter lifetime. A detailed model is a useful tool to improve stack performances, through design improvements. In this paper, a 3D model of a stack composed of 15 circular MCFC, considering heat, mass and current transfer as well as chemical and electrochemical reactions is presented. The model validation is conducted using some preliminary experimental data obtained for an MCFC stack developed in the Fabbricazioni Nucleari laboratories. These results are examined in order to improve the stack configuration. It is shown that power density may be increased of about 20% through double side feeding. In addition, the average temperature gradients in the axial direction are reduced of more than 70%. Significant reductions in the temperature gradients, especially in transversal direction, can be achieved by adjusting the mass flow rate of cathodic gas supplied to the various cell
Numerical computation for parallel plate thermoacoustic heat exchangers in standing wave oscillatory flow
No full textA simplified computational method for studying the heat transfer characteristics of parallel plate thermoacoustic heat exchangers is presented. The model integrates the thermoacoustic equations of the standard linear theory into an energy balance-based numerical calculus scheme. Details of the time-averaged temperature and heat flux density distributions within a representative domain of the heat exchangers
and adjoining stack are given. The effect of operation conditions and geometrical parameters on the heat exchanger performance is investigated and main conclusions relevant for HX design are drawn as far as fin length, fin spacing, blockage ratio, gas and secondary fluid-side heat transfer coefficients are concerned. Most relevant is that
the fin length and spacing affect in conjunction the heat exchanger behaviour and have to be simultaneously optimized to minimize thermal losses localized at the HX-stack
junctions. Model predictions fit experimental data found in literature within 36% and 49% respectively at moderate and high acoustic Reynolds numbers
Algorithmic Debugging of Real-World Haskell Programs: Deriving Dependencies from the Cost Centre Stack
No full textExisting algorithmic debuggers for Haskell require a transformation of all modules in a program, even libraries that the user does not want to debug and which may use language features not supported by the debugger. This is a pity, because a promising ap- proach to debugging is therefore not applicable to many real-world programs. We use the cost centre stack from the Glasgow Haskell Compiler profiling environment together with runtime value observations as provided by the Haskell Object Observation Debugger (HOOD) to collect enough information for algorithmic debugging. Program annotations are in suspected modules only. With this technique algorithmic debugging is applicable to a much larger set of Haskell programs. This demonstrates that for functional languages in general a simple stack trace extension is useful to support tasks such as profiling and debugging
DESIGN & DEVELOPMENT OF PLANAR SOLID OXIDE FUEL CELL STACK
No full textIn the present work, planar anode-supported Solid Oxide Fuel Cell short-stacks have been designed, assembled, tested and characterized. The design of the stacks and its components (frame, housing, interconnect, compressive and bonded seals) required a great attention to the materials properties (i.e. thermal expansion coefficient compatibility, durability, strength and oxidation resistance, conductivity and so on), as well as to the fluid-dynamic analysis focused on flow field and gas distribution. Then, a careful analysis was done based on a multidisciplinary approach to select the stack components materials, geometries, and dimensions; in order to assure a high performing stack at elevated temperatures with cost reduction of materials, parts manufacturing and assembly procedure. The materials selected were: Crofer®22APU for the interconnect and the frame; AISI 316L for bolts and housing; Thermiculite® 866 for the compressive seal placed between the frame and the interconnect plate; Flexible Mica Paper for the compressive seal positioned between the interconnect endplate and the housing; SiO2-CaO-Al2O3-Na2O glass-ceramic sealant for the bonded seal to join the frame with the cell. On the other hand, the stack assembly was focused on the implementation of innovative and simple procedures, which allowed power capacity scale-up in accordance to power requirements. In this work, two different stack configurations were produced: with one cell (for initial testing of the materials and fluid-dynamic selected solutions) and with three cells. It must be mentioned that all developed stacks in this research were assembled with commercial cells "ASC3" from H.C. Starck. Also, calculations at ambient temperature and 800°C were done in the stack compression system to determine the proper tightening torque to be applied: this value was 50N. Although this calculation took into consideration the loss of tightening torque at high temperatures, some marks were found in housing and micas during the stack inspection after disassembly. These marks are a clear indicator of gas leakage. Additionally, a study was carried out related to the effect of the protective Mn1.5Co1.5O4 coating deposited on interconnect surface to prevent the cathode Cr poisoning. This experiment was executed in the stack of one cell configuration. No voltage degradation was observed during the galvanostatic experiment of 360 h at 800°
Divided we stand: Parallel distributed stack memory management
No full textWe present an overview of the stack-based memory management techniques that we used in our non-deterministic and-parallel Prolog systems: &-Prolog and DASWAM. We believe that the problems associated with non-deterministic and-parallel systems are more general than those encountered in or-parallel and deterministic and-parallel systems, which can be seen as subsets of this more general case. We develop on the previously proposed "marker scheme", lifting some of the restrictions associated with the selection of goals while keeping (virtual) memory consumption down. We also review some of the other problems associated with the stack-based management scheme, such as handling of forward and backward execution, cut, and roll-backs
Impaired mobility and Parkinson’s disease
No full textImpaired mobility associated with frequent falls is common in Parkinson's disease (PD) and is the focus of three related papers in this issue. Walking quietly forward, in a straight line across a smooth surface presents people with PD with a basic challenge but when the mobility task is in any way more complex, the level of challenge rises steeply for people with PD.Ashburn et al.'s diary-based survey of over 600 falls suggests that only a handful of activities account for the majority of falls among people with PD, with ‘tripping while walking’ taking first place and ‘falling while turning’ being associated with serious injury. Jones et al.'s qualitative study explores how the distressing uncertainty that one can walk safely from A to B without falling is detrimental to self-esteem and participation in society. In this paper, like that of Ashburn et al., the authors discuss how any kind of ‘multi-tasking’ exacerbates the risk of falling. Stack et al.'s paper describes an analysis of turning to walk from a standing start, an activity that most healthy adults accomplish readily, and discusses why people with PD find turning so notoriously challenging. All three papers address methodological issues around the assessment of mobility, falls and fallers and the authors make innovative suggestions about rehabilitation practice. Together, this survey, interview set and movement analysis add to the existing knowledge base on falls and illuminate difficulties specific to PD. The combination of research methods is successfully drawing researchers to what patients consider the key issues, optimizing the immediate relevance of the research output<br/
The Unified Framework for Sensor Networks: A Systems Approach
No full textSince its introduction in the mid-1970s, the OSI Basic Reference Model (OSI-BRM) has been widely used as a foundation for communication models and standards. While many of these have modified the OSI-BRM for specific communication requirements (protocols such as ZigBee and Fieldbus – used in sensor networks), little structure or standardisation has been developed for other aspects of the hardware/software interface – for example sensing, energy management, actuation or locationing. Such processing is often implemented in the application layer of the communications stack, resulting in an unstructured, top-heavy and confusing stack. Alternatively, processing is performed off-chip or in separate unstructured software. In this paper, we propose the Unified Framework for the structured design and implementation of multiple interfaces on a sensor node. The framework creates unified stacks by connecting individual stacks (containing distinct functionality) via a shared application layer. We present the application of the framework to create a unified stack, structuring both communications and sensing. The process of extending a unified stack for implementing energy management, locationing and actuation is also discussed. The proposed framework establishes a structured platform for the formal design, specification and implementation of sensor and wireless sensor networks
Scope-bounded pushdown languages
No full textWe study the formal language theory of multistack push-down automata (Mpa) restricted to computations where a symbol can be popped from a stack S only if it was pushed within a bounded number of contexts of S (scoped Mpa). We contribute to show that scoped Mpa are indeed a robust model of computation, by focusing on the corresponding theory of visibly Mpa (Mvpa). We prove the equivalence of the deterministic and nondeterministic versions and show that scope-bounded computations of an n-stack Mvpa can be simulated, rearranging the input word, by using only one stack. These results have several interesting consequences, such as, the closure under complement, the decidability of universality, inclusion and equality, and a Parikh theorem. We also give a logical characterization and compare the expressiveness of the scope-bounded restriction with Mvpa classes from the literature
First Impressions of Using Stack Overflow for Education in a Computer Science Bachelor Programme
No full textCommunity Question Answering (CQA) platforms like Stack Over- flow enable gamified and moderated community-driven knowledge creation. We report on our experiences of introducing Stack Over- flow in our Computer Science BSc programme and explore whether these platforms can (1) reduce the workload of lecturers in answer- ing questions in large-scale classrooms, and (2) create a community of learners in the large-scale setting of ∼1600 students.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Software EngineeringComputer Science & Engineering-Teaching Tea
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